In surveying instruments, especially in so-called total stations, it is often required to position a movable unit in the instrument by rotating the movable unit around a horizontal or a vertical axis. The rotation around an axis is normally made via a gear arrangement, either by hand for a manual instrument or by means of an electric motor that is connected to the gear arrangement for an automated instrument. Such a gear arrangement needs to be of very high precision in order to serve its purpose, especially when the instrument is intended to automatically move from one position to another, for instance by means of a tracking arrangement. Such an instrument with a suitable gear arrangement is expensive. Also, in order to meet strict precision requirements, a preloading of the gear arrangement is required. Due to the resulting backlash and hysteresis in the gear arrangement there is a need, in an automated instrument, to have angle sensors both on the motor and on the movable unit in order to obtain a sufficiently stable and exact servo loop for controlling the motor.
The presence of a gear arrangement between the motor and the movable unit prevents a forced movement by hand of the movable unit to a new position, since the gears will block such a movement. In order, however, to allow a forced movement by hand of the movable unit to a new position it has been common to provide a friction clutch between a gear arrangement and the movable unit, thus allowing the movable unit to move while the gear arrangement remains in position.
By using a gear arrangement and two angle sensors as well as a friction clutch at each axis around which the movable unit is to be moved by a motor, the surveying instrument becomes relatively complicated and expensive. There is therefore a need for an improved design in these respects.
As regards general rotation of a movable unit in a surveying instrument it is previously known through US 2002/0005944 A1, now U.S. Pat. No. 6,621,565, to provide an optical transmitter, where a head assembly is rotated around a vertical axis by means of a direct drive electric motor, and where there are means to emit light signals to a distant receiver at certain rotational positions for the head assembly. For an arrangement of this kind there is, however, no need or intention to stop the head assembly in a certain rotational position. Also, there is no need to make possible a forced rotation of the head assembly.